1. Field of the Invention
This present invention relates to a belt transferring type image forming apparatus that is used for electro-photographic copying machines, printers, etc., in particular, for quadruple drums tandem type color copying machines and color printers.
2. Description of the Related Art
Color copying machines, color printers, etc. for image forming using 4 color toners; yellow, magenta, cyan and black (hereinafter, abbreviated as Y, M, C and B, respectively) have been so far developed. As image forming systems using these four color toners, there are four systems as shown below:
1. A system to form an image by superposing 4 color toner images on one photosensitive drum and transfer this image collectively on a sheet of paper;
2. A transfer drum system to hold a sheet of paper on a transferring drum and form 4 color images on the sheet of paper by rotating the transferring drum 4 times;
3. An intermediate transferring body system to form images in four colors on an intermediate transfer body and transfer these images collectively on a paper; and
4. A quadruple drums tandem system with four photosensitive drums arranged in parallel and an image is formed on a sheet of paper during one pass.
Particularly, in case of a quadruple drums tandem type color image forming apparatus, an image is formed separately on each of four photosensitive drums that are arranged in parallel and a multi-transferred color image is formed by one pass of a sheet of paper through the drums and therefore, when compared with the above-mentioned systems 1-3, a time required in the image forming process can be shortened to 1/4. So, this type of image forming apparatus is suited for achieving the high-speed image forming.
In this quadruple drums tandem system, one conveying belt is used and as disclosed in Japanese Published Unexamined Patent Application No. 110343/1994, etc., an apparatus to transfer an image formed on the photo-sensitive drums by a semi-conductive transferring belt and a transferring roller provided on the back of the conveying belt is devised.
However, even for an image forming apparatus using this quadruple drums tandem type image forming system, there are demands not only for outputting color images but also for the mono-color image formation. In the quadruple drums tandem system, the operations of four photosensitive drums and other devices are the same in the mono-color image formation and the full-color image formation. Therefore, even when it is desired to form an image in mono-color, other photosensitive drums not needed are also operated. So, there is such a defect that unnecessary image forming devices including other photosensitive drums, etc. are brought in contact with the conveying belt and consumed and the span of life is shortened. Therefore, the image forming apparatus is provided with a mono-color image forming mode so as to perform the mono-color image forming by bringing the conveying belt into contact with a mono-color photosensitive drum only, while other photosensitive drums are kept stopped to operate, thus ensuring a long life.
However, because one conveying belt is used for quadruple photosenstive drums at present, in order for bringing the conveying belt into contact with the mono-color B photosensitive drum and separating the Y, M and C photosensitive drums, a method to change the shape of the conveying belt is used. When this method is used, the belt tension cannot be precisely controlled and the belt weaves when it is rotated. When the conveying belt weaves, there is caused a problem that the image transferring position to a paper is shifted.